**5. Conclusions**

This work studied the effect of the using annular fuel rods on thermal–hydraulic characteristics of a typical VVER-1000 reactor. The results of the annular fuel showed that the increase in the heated surface improves the margin from the peak temperature of the fuel to melting. The use of Al2O3 nanofluid as a coolant in the VVER-1000 nuclear reactor is also recommended. The results showed that by improving the heat transfer coefficient of Al2O3 nanofluid to increase the concentration of Al2O3 nanoparticles, the coolant temperature increases, and the central fuel temperature decreases. To explain more, there is about 127 K reduction in the maximum value of fuel temperature in the annular fuel with 20 volume percentages of nanoparticles compared to pure water. Furthermore, there is about 13 degrees reduction in the clad temperature with nanofluid compared to the clad temperature with pure water. The results showed the minimum value of DNBR for annular fuel was 1.97. Finally, it is determined that the use of nanofluid with annular fuel rods has the best efficiency in a nuclear reactor. By increasing the heated surfaces in the annular fuel and due to the higher heat transfer coefficient of nanofluids compared to pure water, the coolant flow rate can be reduced. As a result, the reactor core can be more compact, and the plant's capital cost is reduced.
